A position of a selected shift range of an automatic transmission is detected by a TCU (a transmission control unit) based on electrical signals from a position sensor. The TCU drives an indicator to emit the relevant light emitting diodes in accordance with the detected shift range. The position sensor comprises multiple switches, wherein combinations of contacts of the respective switches are changed depending on the selected shift range, namely the respective positions of a movable member which is moved by a shift lever operated by a vehicle driver. The TCU detects the position of the selected shift range operated by the shift lever, based on the combination of “ON” signals and “OFF” signals from the respective switches.
In such a position sensor having multiple switches, the combination of the “ON” signals and the “OFF” signals outputted from the respective switches (hereinafter also referred to as a signal pattern) becomes different from a predetermined signal pattern corresponding to the respective shift ranges, if one of or more than one of the multiple switches become out of order (disconnected or short-circuited). In such a malfunction of the position sensor, the TCU can determine that the malfunction has occurred at the position sensor, when the combination of the outputted signals (the signal pattern) does not coincide with any one of the predetermined signal patterns. Furthermore, in the case that the signal pattern would coincide with one of the predetermined signal patterns in spite that a malfunction (the disconnection or short-circuit of the switches) has actually occurred in the position sensor, then the indication of the shift range by the indicator (e.g. the light emitting diodes) might be erroneously made to the vehicle driver. To the end, the combination of the outputted signals (the signal patterns) is generally so designed that the indication of the shift range (shift position) by the indicator to the vehicle driver is made on a safer side, when a malfunction has occurred in the switches of the position sensor. Namely, the indications by transition positions (“P-R”, “R-N”, of “N-D”) are made in case of the malfunction, in order to avoid indications of the shift ranges “D”, “R”, “N”, or “P”, for example the indications by “D” in stead of “R”, “R” in stead of “D”, or “R” or “D” in stead of “N”. For example, this kind of technologies is disclosed in Japanese Patent Publication No. 2003-294134.
In the above prior art (JP-No. 2003-294134), however, it has a disadvantage in that a detection of the malfunction of the position sensor is delayed, depending on the shift position of the shift lever, the switches of the position sensor in which the malfunction has occurred, and so on. An example is explained with reference to FIGS. 12A and 12B, wherein FIG. 12A shows a map of predetermined signal patterns for a normal operation, whereas FIG. 12B shows signal patterns of outputted signals when a switch S4 is short-circuited. More specifically, in FIGS. 12A and 12B, S1 to S4 are switches of the position sensor, and references “P”, “P-R”, “R” and so on in a vertical direction show the respective shift ranges and transition positions of the shift lever. For example, “P” is the shift range of a parking position, “R” is the shift range of a backward drive position, and “P-R” is a transition position from “P” to “R” range. Black circles indicate “ON” signals and white circles indicate “OFF” signals from the respective switches S1 to S4, depending on the shift position. When the switch S4 is short-circuited, the “ON” signal is always outputted from the switch S4 independently from the position of the shift lever, as shown in FIG. 12B. In this situation, the signal patterns (FIG. 12B) of the position sensor are different from the predetermined signal patterns (FIG. 12A), in the case that the shift lever is positioned at the “P” range”, “N” range, “N-D” position, “2” range, “2-1” position and “1” range. The shift lever is usually positioned at the “P” range, when the engine is started, and therefore, the shift position is moved from the “P” range to the “P-R” position, “R” range and so on, in the order shown in FIGS. 12A and 12B. Although, the signal pattern (FIG. 12B) at the “P” range is different from the signal pattern (FIG. 12A) of the normal operation, the malfunction of the position sensor (the short-circuit of the switch S4) can not be detected at this stage, since the signal pattern (the “ON” signal from the switch S4 in FIG. 12B at the “P” range) also exists in the signal pattern of FIG. 12A in case of the “P-R” position. As in the same manner, even when the shift lever is moved to the “N” range, the signal pattern (the “ON” signals from the switches S1, S2 and S4 in FIG. 12B at the “P” range) also exists in the signal pattern of FIG. 12A in case of the “N-R” position. As above, the malfunction of the position sensor can not be detected, until the shift lever is moved to the “2-1” position. However, the signal pattern at the “2-1” position is the “ON” signals from the switches S3 and S4 and the “OFF” signals from the switches S1 and S2, and this signal pattern does not exist in the signal patterns of FIG. 12A. And therefore, the malfunction of the position sensor can be detected for the first time, when the shift lever is moved to the “2-1” position.